High performance dynamic sense amplifier with active loads

ABSTRACT

A random access read/write MOS memory device consisting of an array of rows and columns of one-transistor memory cells employs a bistable sense amplifier circuit at the center of each column. The sense amplifier is of the dynamic type in that coupling transistors connect the column line halves to the cross-coupled driver transistors. The sources of the driver transistors are connected to ground through a sequentially timed, three step grounding arrangement employing two transistors, one having a dual channel implanted to provide two different threshold voltages. Active load devices connected to the column line halves provide pull-up of the voltage on the one-going column line half to a full Vdd level.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor memory devices and moreparticularly to an improved sense amplifier for an MOS random accessread/write memory.

Semiconductor memory devices of the type made by the N-channelsilicon-gate MOS process and employing one transistor dynamic cells arenow the most widely used in computers and digital equipment. Acontinuing problem in these devices is the sense amplifier which mustdetect the small change in voltage on a digit line caused by a cellbeing addressed. As the number of cells on a digit line increases andthe cell size decreases, the ratio of the storage capacitance to thedigit line capacitance decreases and so the voltage change decreases.The trend toward use of 5 V. power supplies rather than 12 V. alsoreduces the signal level. These factors make the performance of thesense amplifier more critical. Also, the continuing trend toward higherspeeds and lower power dissipation place additional constraints on thesense amplifier design. Examples of prior sense amplifiers are disclosedin U.S. Pat. Nos. 3,909,631, and 4,050,061 issued to N. Kitagawa, U.S.Pat. No. 4,081,701 issued to White, McAdams and Redwine, and pendingapplications Ser. No. 682,687, filed May 3, 1976 (refiled June 30, 1978as Ser. No. 920,755) by Kitagawa and McAlexander, and Ser. No. 691,734,filed June 1, 1976 (refiled June 30, 1978 as Ser. No. 920,756) byKitagawa and White, all assigned to Texas Instruments, as well asarticles in Electronics Magazine, Sept. 13, 1973 at pp. 116-121, Feb.19, 1976 at pp. 116-121, and May 13, 1976 at pp. 81-86 and U.S. Pat. No.4,061,999. The prior sense amplifiers have not been adequate for newdesigns of MOS RAMs of very high density--64K bits, operating on asingle 5 V. supply with access times of 100 to 150 nsec. or faster.

It is the principal object of this invention to provide an improvedsense amplifier for a high speed MOS RAM, particularly for a very densearray of one-transistor cells. Another object is to provide a senseamplifier which may be used in a dense MOS memory which operates from alow voltage supply such as 5 V.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a random accessread/write MOS memory device consisting of an array of rows and columnsof one-transistor memory cells employs a bistable sense amplifiercircuit at the center of each column. The sense amplifier is of thedynamic type as seen in U.S. Pat. No. 4,061,999 in that couplingtransistors connect the column line halves to the cross-coupled drivertransistors. The sources of the driver transistors are connected toground through a sequentially timed, three step grounding arrangementemploying two transistors, similar to application Ser. No. 920,755, onetransistor being dual channel implanted to provide two differentthreshold voltages. Active load devices as in U.S. Pat. No. 4,081,701,connected to the column line halves, provide pull-up of the voltage onthe one-going column line half to a full Vdd level.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asother features and advantages thereof, will be best understood byreference to the detailed description which follows, read in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is an electrical diagram in block form of a semiconductor dynamicmemory device which may use the sense amplifiers of the invention;

FIGS. 2a-2g are graphic representations of voltage vs. time or otherconditions vs. time existing for various parts of the device of FIG. 1;

FIG. 3 is an electrical schematic diagram of a part of the device ofFIG. 1 showing the sense amplifier of the invention in detail in amemory array; and

FIGS. 4a-14l are graphic representations of voltage vs. time existing atvarious parts of the circuit of FIG. 3.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT

Referring to FIG. 1, a memory device which may utilize the senseamplifiers of the invention is illustrated in block diagram form. Thisis a random access, read/write memory of the dynamic type, made by anN-channel, self-aligned, silicon gate MOS process. All of the memorydevice of FIG. 1 is included in one silicon chip of about 1/30 of asquare inch in size which usually would be mounted in a standarddual-in-line package having sixteen pins or terminals. The deviceincludes in this example an array 10 of 65,536 memory cells, with thearray split into two halves 10a and 10b of 32,768 cells each, in aregular pattern of 256 rows and 256 columns. Of the 256 row or X lines,there are 128 in the array half 10a and 128 in the half 10b. The 256column or Y lines are each split in half with one half being in each ofthe halves 10a and 10b. There are 256 sense amplifiers 11 in the centerof the array; these are differential type bistable circuits madeaccording to the invention, and each one is connected in the center of acolumn line. Thus 128 memory cells are connected to each side of eachsense amplifier by a column line half. The chip requires only a single 5V. supply Vdd, along with a ground terminal Vss. No internal charge pumpis needed because no substrate bias is employed.

A row or X address decoder 12, split into two halves, is connected bysixteen lines 13 to eight address buffers or latches 14 via outputcircuits 15. An eight-bit X address is applied to inputs of the addressbuffers 14 by eight address input lines 16. The X decoder 12 functionsto select one of the 256 row lines as defined by an eight bit address onthe input terminals 16; if the selected row line is in the half 10b ofthe cell array then a row of dummy cells 17 on the opposite side of thesense amplifiers 11 is also activated, while if a line in the half 10ais selected then a row of dummy cells 18 is activated. The addresssignals on the input lines 16 are multiplexed; the Y address is alsoapplied to these input lines and is latched into a set of eight buffers19, from which it is applied to column decoders 20, 21 and 22 via outputcircuits 23 and lines 24. A one-of-64 selection is made by the columndecoders 20 and 21, so that one group of four columns is connected tosets of four data and data bar lines 25 and 26, based on six bits of theeight bit Y address. A one-of-four decoder 22 selects one pair of thefour pairs of lines 25 and 26, based on two bits of the eight bit Yaddress, and connects the selected pair to a data I/O control circuit 27via a pair of lines 28. A single bit data input is applied by an inputterminal 30 to a data input latch 31, and the output of this latch iscoupled to the data I/O control 27. The latch 31 may be of the samecircuit design as the address latch circuit 14. One-bit data output isconnected from the data I/O control 27 through a buffer 32 to a data outterminal 33.

The X address must appear on the inputs 16 when a row address strobesignal, referred to as RAS, is applied to an input 34. Likewise, the Yaddress must appear during a column address strobe signal CAS on aninput 35. A read/write control W on an input 36 is the other controlsignal for the device. These three inputs are applied to clock generatorand control circuitry 37 which generates a large number of clocks andcontrol signals to define the operation of various parts of the device.Whe RAS goes low as seen in FIG. 2a, clocks derived from RAS cause thebuffers 14 to accept and latch the eight bits then appearing on theinput lines 16. When CAS goes low as seen in FIG. 2b then clocksgenerated in the circuitry 37 cause the buffers 19 to latch on the Yaddress on the inputs 16. The row and column addresses must be validduring the time periods shown in FIG. 2c. For a read cycle, the W signalon input 36 must be high during the period seen in FIG. 2d, and theoutput on the terminal 33 will be valid during the time seen in FIG. 2e.For a write cycle, the W signal must be low as seen in FIG. 2f and thedata-in bit must be valid during the time seen in FIG. 2g.

In FIG. 3, a portion of the cell array is shown in schematic form. Fouridentical sense amplifiers 11 are positioned at the center of the array,connected to four column line halves 38a or 38b. Sixty-three other setsof four sense amplifiers and column lines are included in the array.Connected to each column line half 38a or 38b are 128 one-transistorcells each having a storage capacitor 40 and a transistor 41. The cellsare of the type described in pending U.S. Patent Application Ser. No.648,594, filed Jan. 12, 1976 and Ser. No. 722,841, filed Sept. 13, 1976,by C-K Kuo, both assigned to Texas Instruments, or U.S. Pat. No.4,012,757. Row lines 43 are connected to the gates of all of thetransistors 41 in each row; there are 256 identical row lines 43 in thearray. Also connected to each column line half 38a or 38b is a dummycell 17 or 18 which consists of a storage capacitor 44 and transistors45. The gates of all dummy cells in a row are connected to a line 46 or47. When the X address selects one of the lines 43 on the left, theassociated transistor 41 is turned on to connect the capacitor 40 forthis selected cell to the column line half 38a, while at the same timethe dummy cell select line 47 on the opposite side is activated,connecting the capacitor 44 in one of the cells 18 to the column linestorage cell capacitance 40. The dummy cell is precharged to a logiczero before every active cycle.

According to the invention, the improved sense amplifier consists of abistable circuit havng a pair of driver transistors 50 and 51, each withits gate connected to the drain 52 or 53 of the other to provide across-coupled flip-flop. The drains 53 and 53 are connected to nodes 54and 55 at the ends of the lines 38a and 38b through the source-to-draincurrent paths of a pair of coupling transistors 56 and 57. The gates ofthe transistors 56 and 57 are both connected to a source of a clockvoltage Ptr, seen in FIG. 4j, which is above Vdd for most of the cyclethen drops to Vdd during the active part of a cycle. The nodes 54 and 55and column line halves 38a and 38b are precharged through thesource-to-drain current paths of a pair of transistors 58 and 59connected to a voltage source Psp; this voltage source, shown in FIG.4g, is Vdd during the precharge part of the cycle, drops to anintermediate level, then drops to zero during the active part of thecycle. The gates of the transistors 58 and 59 are connected to a clockvoltage Psl seen in FIG. 4h.

The sources of the driver transistors 50 and 51 are connected togetherat a node 60, and this node 60 is connected by a line 61 to the samenode in all of the 256 sense amplifiers 11 in the array. The line 61 isconnected to a transistor 62 and a dual channel transistor 63 and 64which function as grounding paths. The gate of the transistor 62 isconnected to a clock Psbl seen in FIG. 4b, and the common gate of thedual transistors 63 and 64 is connected to a clock Psb2 seen in FIG. 4c.This grounding arrangement is similar to that of application Ser. No.682,687, filed May 3, 1976, refiled June 30, 1978 as Ser. No. 920,755,assigned to Texas Instruments. Instead of using separate clock sourcesfor the dual transistors 63 and 64, however, an important feature is theuse of a single clock source. The two current paths of the dualtransistor 63 and 64 turn on at different times because the channel areaof the transistor 64 is ion implanted to raise its threshold so that itturns on later than the transistor 63 even through the same clock isapplied to its gate. The dual transistor 63 and 64 (actually one largetransistor with different channel implants) is much larger than thetransistor 62, in channel width to length ratio.

As described thus far, operation of the sense amplifier is similar tothat of U.S. Pat. No. 4,061,999, as used in the 4027 and 4116 dynamicRAM devices. The column line halves 38a and 38b along with nodes 54 and55 are precharged to near Vdd during the precharge part of an operatingcycle when both Psp and Psl are high. At this time Ptr is high so thenodes 52 and 53 are precharged also. The transistors 50 and 51 are offbecause the transistors 62-64 are all off, Psb1 and Psb2 being low.After Ps1 has gone low, turning off the transistors 58 and 59, andbefore Psb1 goes high, an X address reaches one of the lines 43 at thesame time that one of the dummy cell address lines 46 or 47 isactivated. This causes an imbalance in the voltage on the nodes 54 and55, and the same differential is coupled to the nodes 52 and 53 becausethe voltage Ptr is higher than Vdd. The nodes will separate no more thanperhaps fifty milivolts at this point. Then, when Psb1 goes high and thesmall transistor 62 turns on, the sensing operation is initiated and theanodes separate more as the bistable circuit including the transistors50 and 51 goes toward a stable condition with one transistor conductingand the other cut off. A slight delay from Psb1, the clock Psb2 goeshigh to complete the sensing operation by latching the bistable circuitand obtaining a good one/zero set on opposing digit lines. By capacitor65 along with the parasitic capacitances of the transistors 56 and 57,the voltage Ptr is dynamically level shifted from greater than Vdd downto Vdd; the drop in voltage on the node 60 toward Vss as Psb1 then Psb2go high is coupled to the gates of the transistors 56 and 57. Thisresults in maintenance of a low conductivity channel between nodes 54and 52 and between nodes 55 and 53, through the transistors 56 and 57.While latching is initially occurring between the transistors 50 and 51,the column lines 38a and 38b are capacitively isolated from the sensingnodes 52 and 53. When one or both of the nodes 52 and 53 falls by one Vtbelow Ptr, then the channel conductance will increase and the digitlines will follow in accordance with the now determined and latchedstate of the bistable circuit. Ptr is clamped at Vdd just after Psb2goes high.

According to the invention, an active pull-up circuit is employed toallow storage of a full Vdd level. This circuit comprises a pair ofpull-up transistors 66 and 67 connecting the nodes 54 and 55 to Vdd,along with control transistors 68 and 69 connecting the gates of thetransistors 66 and 67 to the nodes 54 and 55, and capacitors 70 and 71connecting the gates to a boosting clock Pb occurring after Psb2. Thegates of the transistors 68 and 69 are connected to a trap voltage Vtrwhich stays at a level of about 1Vt below Vdd during the active part ofthe cycle then at Vdd during the precharge part. The details ofoperation of the active pull-up circuit will be described below.

After the sensing operation is essentially completed and Psb2 has comeon to render first the low threshold transistor 63 then after a slightdelay the higher threshold transistor 64 conductive, a definite logicone and logic zero are set up on the column lines 38a and 38b. Then,about four ns. after Psb2 goes high, the selected X line is slowlyboosted to a level of Vdd+Vt to permit restoration of a full Vdd levelin the capacitor 40 for the selected cell. The voltage on the dummy cellselect line 46 or 47 is not boosted because the dummy cell capacitor 44never stores a one; it is always discharged or at logic zero. At thesame time the X line 43 is being boosted, the clock Pb goes high toactivate the active load circuits. The clock Pb causes level shift ateither node 72 or 73 via the gated capacitors 70 and 71. Only one ofthese nodes will have retained a logic one because the column lines willbe near the one/zero set at this time; conduction through thetransistors 68 or 69 for the zerogoing side will discharge node 72 or 73and cause the gated capacitor 70 or 71 to exhibit very littlecapacitance so Pb will not charge the node 72 or 73 for this side. Theother node 72 or 73 which retained a one, at near Vdd, will be shiftedto greater than Vdd thereby allowing this column line half to be pulledback up to Vdd through transistor 66 or 67. At the same time as Pboccurs, the clock Psp is pulled to Vss.

Selection of one group of four of the 256 column lines 38a and 38b by aPyh voltage occurs a slight delay from when Psb2 goes high. This ensuresquiet sensing because only sensing signals occur in the vicinity of thesense amplifier during the critical time of the sensing operation. Theone-of-64 column decoder 20 and 21, physically located in the spacebetween the sense amplifiers 11 and the data and data bar lines 25 and26, produces only one Pyh signal on a line 74 to activate only one setof four transistors 75 coupling nodes 54 to lines 25 and one set of fourtransistors 76 coupling nodes 55 to lines 26. The remaining sixty-threesets of sense amplifiers 11, although operative for refresh on everyread or write cycle, will not be coupled to the data and data bar linesbecause the Pyh signal on the line 74 will be low for these.

Upon completion of the active portion of a read or write cycle, theprecharge portion of the cycle is activated by RAS going high. Theselected X line 43 and dummy cell line 46 or 47 are first pulled low toisolate the selected bit cells and dummy cells. Ps1 goes high towardVdd, shorting the column lines 38a and 38b to Psp, rapidly equalizingthe voltages on the anodes 54 and 55 via Psp, through the transistors 58and 59 to a voltage slightly above Vss. A slight overlap between turningon transistor 58 and 59 by Psl and bringing Psp high promotes rapidequalization at near Vss. Then, as Psp is pulled back to a full Vdd, andthe column lines 38a and 38b also are pulled back up to Vdd, Psl isboosted above Vdd, promoting equalization as the node 54 and 55 voltagesincrease. The capacitors 44 in the dummy cells are discharged to Vss byPsd going to Vdd. The clocks Psb1 and Psb2 are pulled low just prior toequalization of the column lines 38a and 38b. The subsequent prechargingof the column lines 38a and 38b and the nodes 52, 53 and 6o boost Ptr togreater than Vdd through transistors 56 and 57. Pb is pulled low alsoprior to equalization so that no interference is injected into the senseamplifier precharge balance operation. Vtr is precharged to Vdd and atthe start of the active portion of the cycle, Vtr is pulled to less thanVdd to ensure that the active loads remain totally inactive until one ofthe column lines 38a or 38b falls to Vdd-2Vt and also ensures thatadditional parasitic capacitances on the nodes 72 and 73 are not seen bythe column lines 38a and 38b until after the sense amplifier latchinghas occurred.

While this invention has been described with reference to anillustrative embodiment, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiment, as well as other embodiments of the invention, will beapparent to persons skilled in the art upon reference to thisdescription. It is, therefore contemplated that the appended claims willcover any such modifications or embodiments as fall within the trucescope of the invention.

What is claimed is:
 1. A sense amplifier for a semiconductor memorydevice of the type having an array of rows and columns of memory cellswith each column split into two column line halves and sense amplifiersconnected to pairs of sense nodes at the ends of each pair of columnline havles, comprising:a pair of cross-coupled driver transistors, apair of coupling transistors, at least one grounding transistor and apair of pull-up transistors, each of the transistors having a currentpath and a control electrode, means connecting the current path of eachof the coupling transistors in series with the current path of aseparate one of the driver transistors between one of the sense nodesand a grounding node and connecting the current path of said at leastone grounding transistor between the grounding node and referencepotential, the current paths of the pull-up transistors being connectedseparately between the sense nodes and a supply of a given voltagelevel, means for precharging the sense nodes to said voltage level priorto an active operating cycle, means for addressing a selected row of thememory cells in the array at a given time in the beginning of saidoperating cycle at a voltage of said voltage level, means for applying aclock voltage to turn on said at least one grounding transistor at afirst time subsequent to the said given time in the beginning of anactive operating cycle, coupling means separately connecting the controlelectrodes of the pull-up transistors to the sense nodes, the couplingmeans being conductive only for a given voltage differential during theactive operating cycle, means to boost the voltage on the controlelectrode of one of the pull-up transistors to higher than said voltagelevel at a time in the active operating cycle just following the secondtime, means to maintain the voltage on the control electrodes of thecoupling transistors at higher than said voltage level prior to thebeginning of active operating cycle and shifting the level of saidvoltage level during the active operating cycle, and means foraddressing said selected row of the memory cells at a voltage higherthan said voltage level at a time during said active cycle after saidfirst time.
 2. A sense amplifier according to claim 1 wherein the memorycells are of the dynamic type each having one transistor and one storagecapacitor.
 3. A sense amplifier according to claim 2 wherein all of saidtransistors are insulated gate field effect ransistors, the current pathof each of the transistors being a source-to-drain path and the controlelectrode being a gate.
 4. A sense amplifier according to claim 3wherein the coupling means are insulated gate field effect transistors,and a voltage is applied to the control electrodes thereof to define theconductivity thereof.
 5. A sense amplifier according to claim 4 whereinsaid voltage differential is about two threshold voltages for the fieldeffect transistors.
 6. A sense amplifier according to claim 1 whereinsaid at least one grounding transistor includes a second transistorhaving a dual channel current path, one part of the channel turning onprior to the other part of the channel.
 7. A sense amplifier accordingto claim 6 wherein the dual channel is created by different ionimplants.
 8. A sense amplifier according to claim 1 wherein the voltageon the control electrodes of the coupling transistors is higher thansaid voltage level until said first time in an active operating cycle.9. A sense amplifier according to claim 1 wherein the means forprecharging comprises a pair of precharge transistors having currentpaths separately connecting a precharge node to said sense nodes.
 10. Asense amplifier according to claim 9 wherein the precharge node variesin voltage from said voltage level to an intermediate level then toreference potential during an active operating cycle then slowly risesback to said voltage level after an active operating cycle.